Machine dishwashers are widely used to clean soiled dishes, cooking utensils and other containers for serving and preparing food, such as plates, cups, glasses, silverware, pots, pans, etc., generically referred to as "dishes". While the construction and composition of these items vary widely, most usually have glossy, solid surfaces on which the presence of dried water spots is readily noticeable. These dried water spots are aesthetically unappealing and thus methods and compositions for reducing their number and size are of great concern to the detergent industry as well as to the consumer.
Typical machine dishwashers operate by subjecting food soiled dishes and the like to alternating wash and rinse cycles inside a closed washing chamber. Spray nozzles inside the chamber direct powerful streams of hot wash liquor and rinse water onto the soiled dish surfaces. A considerable amount of food residue is thereby removed by the force exerted by these pressurized water streams. There is, however, a substantial amount of food residue which resists dislodgement by the water jets. It is known that by adding certain detergent compositions to the wash water, a much greater quantity of food soil can be removed from soiled article surfaces during the washing cycle. These machine or "automatic" dishwasher detergent compositions as they are known dramatically increase the cleaning efficiency of machine dishwashers primarily by alkaline cleaning action and through the emulsification and dispersion of food residue which otherwise clings to dish surfaces despite the action of wash water jets.
While food soils vary greatly in composition, generally food residue which remains on dish surfaces contains an organic component which often includes a mixture of various proteins. Since the make-up of typical food residue is known, detergent compositions are formulated such that their various components are effective in breaking down food soils during the wash cycle of automatic dishwashers. The emulsification of food soil is most often achieved through the use of surface active agents or "surfactants", as they are known. Surfactants not only help remove food residue by emulsification, they also provide cleaning power through wetting, foaming, dispersing and penetrating actions. It will be recognized by those skilled in the art that there are many different types of surfactants suitable for use in automatic dishwasher detergents and that low foaming, non-ionic surfactants are especially suited for use in these compositions. For example, low foaming, fatty alcohol ethoxylates and ethylene oxide/propylene oxide block polymers are widely used in the manufacture of machine dishwashing detergents.
Automatic dishwashing detergents also contain detergent builders such as complex phosphates, carbonates, sulfates and silicates which compliment the detersive action of sufactants. For example, by sequestering certain metallic ions which are present in most water sources, phosphates reduce hard water film formation caused by the deposition of metallic precipitates. Moreover, alkalinity builders provide alkaline cleaning power which is particularly important in automatic dishwashing compositions since, by its very nature, automatic dishwashing does not provide the mechanical action of hand dishwashing. Since physical contact with the wash liquor by the consumer does not occur when dishes are cleaned in a machine dishwasher, alkalinity builders are especially suitable for use in automatic dishwashing detergents.
Bleaches can be used in connection with these other dishwashing detergent composition components to remove food residue by breaking down the food residue to simpler components. However, the cleaning action of bleaches differs somewhat from most other dishwashing composition components in that bleaches clean by breaking chemical bonds within molecules rather than the physical bonds which exist between molecules. Although some bleaches are reducing agents, typical bleaches employed in dishwashing compositions are oxidizing agents. Basically, oxidizing bleaches cause the oxidation of chemical bonds, thereby reducing large molecules to smaller units. The smaller units can then be more easily removed during the dishwashing process. The most widely used oxidizing bleaches, particularly in dishwashing compositions, are dry chlorine bleaches. Many compositions use such hypochlorite generators as sodium dichloroisocyanurate and chlorinated trisodium phosphate. Chlorine bleaches are strong oxidizing agents but their usefulness is limited by their tendency to cause unwanted oxidation of other wash compound components and by their brief shelf life. Chlorine bleaches are such strong oxidizing agents that great care must be taken to ensure that the other components with which they are mixed are not adversely affected during storage and, moreover, during use. Due to these and other drawbacks inherent in the use of chlorine bleaches in dishwashing compositions, many formulators have instead proposed that oxygen bleaches be used as the principal oxidizing agent in dishwashing compositions. Oxygen bleaches not only possess greater potential compatibility with other dishwashing composition components, they exhibit much greater stability during storage. While the most frequent oxygen bleach proposed for use is sodium perborate, the use of many inorganic peroxides, such as sodium percarbonate, potassium monopersulfate, and hydrogen peroxide has also been proposed.
It is evident then that the process by which food residue is removed from dish surfaces is the result of various physical and chemical interactions which take place during the washing and rinsing cycles inside the wash chamber. It is theorized that food particles which remain on dish surfaces after final rinse promote water droplet formation and, ultimately, form troublesome dried water spots. These minute food particles serve to anchor droplets of water which would otherwise sheet off of dish surfaces. In particular it is thought that bits of protein residue have a tendency to adhere to article surfaces such that water droplets accumulate around the particles. It follows that it is these protein-anchored water droplets which dry into unsightly water spots. It will be apparent to those skilled in the art that a droplet of water contains a number of minerals and other substances which, when the water evaporates, leaves a residue corresponding roughly in size and shape to the original water droplet. This dried residue causes light to diffract at the water spot in a manner different from that of the surrounding area, thus making the spot visible. It would according to theory, then, seem highly desirable to eliminate as many of these particulate protein masses as possible in order to reduce water droplet accumulations which are believed to be the source of unwanted water spotting of dishes and the like.
It is known in the art that certain proteolytic enzymes are capable of reducing the amount of particulate protein soil on dish surfaces during machine dishwashing operations. In particular, proteases are effective in the removal of protein food soil by virtue of their ability to catalyze the hydrolysis of protein peptide linkages. Proteins, which are very high molecular weight compounds, are thereby converted into smaller peptide units which are more easily removed from article surfaces during the cleaning process. Automatic dishwashing compositions have thus been formulated in the past which contain proteases such as that disclosed in U.S. Pat. No. 4,101,457 to Place et al, entitled "Enzyme-Containing Automatic Dishwashing Composition". Due to a long-held belief in the industry, however, that proteolytic enzymes are incompatible with bleaches in solution, these prior art attempts to utilize proteolytic enzymes in the breakdown of protein soil have generally not included the addition of a bleaching agent in the final composition. Thus, these known compositions have achieved no net reduction of water spotting. Any benefit produced by the protease is negated by the lack of bleach cleansing power in the composition.
Proteolytic enzymes are themselves proteinaceous molecules. Therefore, it has been generally held that when these enzymes are brought in contact with bleach, the enzymes are oxidized into simpler components, thus losing their ability to catalyze the breakdown of peptide bonds. That is, it has been thought that unless the proteolytic enzymes are protected somehow from oxidation by the bleach, they are unable to perform their catalytic function. This widely held belief has resulted in numerous attempts to isolate bleaches from proteass both during storage of compositions containing both of these ingredients as well as during the wash cycle. For example, the attempts have made to microencapsulate bleach to delay its oxidizing action in the wash liquor until after the proteolytic enzymes have had an opportunity to catalytically break down protein soils.
I have discovered that despite the teachings of the prior art that bleaches and proteolytic enzymes are essentially incompatible, a mixture containing an oxygen bleach, a protease and a polyacrylate provides excellent reduction of water droplet formation on washed articles and thus prevents unwanted water spotting during the automatic dishwashing operation. Rather than producing an antagonistic reaction, I have observed that by combining an oxygen bleach, a protease and a polyacrylate along with standard detergent components, an unexpected synergistic action is produced which yields a high degree of water spot prevention. Also, I have discovered an automatic dishwashing detergent composition which contains both an oxygen bleach and a proteolytic enzyme that produces good water spot reduction when used in machine dishwashers.